[blurbery]

What this changes

Adds the Discord invite to the README, in the same places Sponsor already shows up:

• a Discord badge in the badge row;
• a Discord link in the top links row;
• a line under Contributing for questions and a chat.

How I tested it

README only, no code changes. I checked that the invite works and is set never to expire, and that the badge and links point at it.

Release notes

BEGIN_COMMIT_OVERRIDE
END_COMMIT_OVERRIDE

AI disclosure (required)

• No AI tools were used for any part of this change.
• AI tools were used for this change, as described below.

AI tools used and what they did:

I designed and directed this change (a Discord badge, a link in the top links row and a line under Contributing) and used Claude Opus to help write the README text.

Checklist

• I've read CONTRIBUTING.md.
• The title follows Conventional Commits, like fix: keep posters on Jellyfin 12.
• I understand every line of this change and can explain it.
• New source files have the CineSets header, and the unit tests pass.
• Nothing private is included (server addresses, API keys, user names, logs).